Many flexible printed wiring boards have been used in small-low-profile electronic devices and flat electronic devices. Recently, further increase of the demand for increase in density and decrease in size of electronic devices lead to increased need for lamination of flexible printed wiring boards while requirements for flexible printed wiring boards increase in quality and cost.
For production of a laminated flexible printed wiring board, it is needed to laminate a core material having an insulating layer (first insulating layer), a polyimide film or the like and a conductor wiring (first conductor wiring), an insulating layer (second insulating layer), and a conductor wiring (second conductor wiring). The second insulating layer and the second conductor wiring can be formed for example by a method of using a metal foil-bonded resin sheet (see Patent Document 1).
The metal foil-bonded resin sheet has a structure consisting of a metal foil and a resin layer in a half-hard condition that are laminated to each other. The resin layer of the metal foil-bonded resin sheet is placed on a core material and the metal foil-bonded resin sheet and the core material are heated under pressure in the state. It leads to fluidization of the resin layer, filling the resin layer in the space between the first conductor wirings and also the resin layer hardens thermally, forming the second insulating layer. During the pressure molding under heat, an elastic cushioning material is placed on the metal foil-bonded resin sheet so that the resin layer easily moves when fluidized, and the metal foil-bonded resin sheet and the core material are pressurized through the cushioning material. The metal foil is then subjected, for example, to etching treatment, to form the second conductor wirings.
Under recent increasing need for further increase in density of flexible printed wiring boards, it is desired that finer conductor wirings and finer components are mounted on a multi-layer flexible printed wiring board.